1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication process thereof, and particularly to a static induction (SI) thyristor and a fabrication process thereof.
2. Description of the Related Art
FIGS. 1A and 1B are cross-sectional views illustrating a conventional static induction thyristor and a fabrication process thereof, which were proposed by the present inventor.
This type static induction thyristor 100 was conventionally fabricated in the following manner.
As illustrated in FIG. 1A, an N.sup.- substrate 10 and an N.sup.+ substrate 20, which have been subjected to mirror polishing on at least sides to be joined to each other, are first provided.
A plurality of recesses 42 is then defined in the top 14 of the N.sup.- substrate 10 by a photolithography process. Thus, projections 44 are defined between the recesses 42.
A P.sup.+ layer 12 is then formed on the underside of the N.sup.- substrate 10 by an impurity-diffusing process.
Further, boron, which is a P-type impurity, is selectively diffused from the top side of the N.sup.- substrate 10 therein, whereby P.sup.+ -gate regions 52 are each formed in the whole exposed surface ranging from the bottom 43 to lower side portions of the recesses 42 defined in the N.sup.- substrate 10.
Thereafter, the N.sup.- substrate 10 and the N.sup.+ substrate 20 are subjected to ultrasonic cleaning with an aqueous solution of sulfuric acid and hydrogen peroxide to remove organic substances and metals on the substrates.
The N.sup.- substrate 10 and the N.sup.+ substrate 20 are then washed with purified water and dried by a spin dryer at room temperature.
The N.sup.- substrate 10 and the N.sup.+ substrate 20 are then heated at about 400.degree.-1000.degree. C. in a hydrogen atmosphere in a state that the tops 45 of the projections 44 between the recesses 42 of the N.sup.- substrate 10 have been brought into contact with the underside of the N.sup.+ substrate 20 as illustrated in FIG. 1B, whereby the N.sup.- substrate 10 and the N.sup.+ substrate 20 are joined to each other.
Further, an anode electrode 92 and a cathode electrode 94 are formed on the underside of the P.sup.+ layer 12 formed on the underside of the N.sup.- substrate 10 and the top of the N.sup.+ substrate 20, respectively.
In the static induction thyristor 100 thus fabricated, the P.sup.+ layer 12 functions as an anode, while the N.sup.+ substrate 20 functions as a cathode. The N.sup.- substrate 10 functions as an N base. The P.sup.+ -gate regions 52 function as a gate which controls an anode current flowing between the anode electrode 92 and the cathode electrode 94.